A fuel cell stack causes a power-generation reaction when a cathode gas and an anode gas are respectively supplied to the front and the back of an electrolyte membrane. When the electrolyte membrane is in a proper wet state, the fuel cell stack efficiently causes the power-generation reaction. Depending on outside-air conditions and operating conditions, however, the electrolyte membrane is brought into an over-dry state. Therefore, in JP 2002-352827 A, the wet state is detected based on an impedance of the fuel cell stack. Then, when the over-dry state is determined, a flow rate of the cathode gas is reduced. In this manner, the electrolyte membrane is prevented from being brought into the over-dry state.